1. Field of the Invention
Present invention refers to antennas for radio communications with broad bandwidth.
2. Description of the Related Art
Antennas for radio communication consist of one or more radiating elements. In case of multiple elements, these are connected in a specifically designed array in order to get the required radiation pattern.
The radiating element can be of different types, e.g. a monopole, a dipole, a patch etc. Each of these types has different advantages and drawbacks.
Dipoles are suitable to use in low loss antennas. The half-wave dipole is a straight conductor that is one half wavelength long, generally fed in the middle. In practice, most dipoles are built with two straight conductors that are a quarter wavelength long. In order to feed this design properly, the signals applied on each of the two wires must have the same amplitude and be in counter-phase described as + and −, see FIG. 1. This type of feeding is called Balanced.
For a sector antenna a ground plane is generally placed behind the dipole at a distance of approximately a quarter wavelength. This enhances the antenna directivity by reducing its radiation towards the back.
Generally, transmission lines deliver an unbalanced signal. To transform this signal into a balanced one, one solution is to use a Balun transformer, abbreviated balanced-unbalanced.
Today it is often required to cover more than one cellular and UMTS of 1920-2170 MHz. For this purpose, it is desirable to use dipoles covering the complete bandwidth of 1710-2170 MHz. For such a dipole to be usable, it needs to provide a stable radiation pattern as well as being impedance matched over the whole bandwidth.
The problem with state-of-the-art dipoles is that they do not combine wide bandwidth with low loss. In some cases, complex matching networks are used to improve the broadband impedance matching of the dipoles.